Then depart from the place whence all the people depart and ask Allah for His forgiveness. Truly, Allah is Oft-Forgiving, Most-Mercifu (199)

This Ayah contains Allah's order to those who stand at 'Arafat to also move on to Al-Muzdalifah, so that they remember Allah at Al-Mash'ar Al-Haram. Allah commands the Muslim to stand with the rest of the pilgrims at 'Arafat, unlike Quraysh who (before Islam) used to remain in the sanctuary, near Al-Muzdalifah, saying that they are the people of Allah's Town and the servants of His House. Al-Bukhari reported that 'A'ishah said, "Quraysh and their allies, who used to be called Al-Hums, used to stay in Al-Muzdalifah while the rest of the Arabs would stand at 'Arafat. When Islam came, Allah commanded His Prophet ﷺ to stand at 'Arafat and then proceed from there. Hence Allah's statement:

مِنْ حَيْثُ أَفَاضَ النَّاسُ

(...from the place whence all the people depart.)

This was also said by Ibn 'Abbas, Mujahid, 'Ata', Qatadah and As-Suddi and others. Ibn Jarir chose this opinion and said that there is Ijma' (a consensus among the scholars) for it.

Imam Ahmad reported that Jubayr bin Mut'im said, "My camel was lost and I went out in search of it on the day of 'Arafah, and I saw the Prophet ﷺ standing in 'Arafat. I said to myself, 'By Allah he is from the Hums. What has brought him here?'" This Hadith is also reported in the Sahihayn.

Al-Bukhari reported that Ibn 'Abbas said that 'depart' mentioned in the Ayah refers to proceeding from Al-Muzdalifah to Mina to stone the pillars. Allah knows best.

Allah said:

 وَاسْتَغْفِرُوا اللَّهَ إِنَّ اللَّهَ غَفُورٌ رَّحِيمٌ

(...and ask Allah for His forgiveness. Truly, Allah is Oft-Forgiving, Most-Merciful.)

Allah frequently orders remembrance of Him after acts of worship are finished. Muslim reported that Allah's Messenger ﷺ used to ask Allah for His forgiveness thrice after the prayer is finished. It is reported in the Two Sahihs that the Prophet ﷺ encouraged Tasbih (saying Subhan Allah, i.e., Glorified is Allah), Tahmid (saying Al-Hamdu Lillah, i.e., praise be to Allah) and Takbir (saying Allahu Akbar, i.e., Allah is the Most Great) thirty-three times each (after prayer).

Ibn Marduwyah collected the Hadith that Al-Bukhari reported from Shaddad bin Aws, who stated that Allah's Messenger ﷺ said:

سَيِّدُ الْاِسْتِغْفَارِ أَنْ يَقُولَ الْعَبْدُ: اللَّهُمَّ أَنْتَ رَبِّي، لَا إِلٰهَ إِلَّا أَنْتَ، خَلَقْتَنِي وَأَنَا عَبْدُكَ، وَأَنَا عَلى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ، وَأَبُوءُ بِذَنْبِي، فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إلَّا أَنْتَ، مَنْ قَالَهَا فِي لَيْلَةٍ فَمَاتَ فِي لَيْلَتِهِ دَخَلَ الْجَنَّــةَ، وَمَنْ قَالَهَا فِي يَوْمِهِ فَمَاتَ دَخَلَ الْجَنَّــةَ

(The master of supplication for forgiveness, is for the servant to say: 'O Allah! You are my Lord, there is no deity worthy of worship except You. You have created me and I am Your servant. I am on Your covenant, as much as I can be, and awaiting Your promise. I seek refuge with You from the evil that I have committed. I admit Your favor on me and admit my faults. So forgive me, for none except You forgives the sins.' Whoever said these words at night and died that same night will enter Paradise. Whoever said it during the day and died will enter Paradise.)

Furthermore, it is reported in the Two Sahihs that 'Abdullah bin 'Amr said that Abu Bakr said, "O Messenger of Allah! Teach me an invocation so that I may invoke (Allah) with it in my prayer. He told me to say:

قُلْ: اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ، وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ

(Allahumma inni zalamtu nafsi zulman kathiran, wa la yaghfirudh-dhunuba illa Anta faghfirli maghfiratan min 'indika, war-hamni innaka Antal-Ghafur-ur-Rahim (O Allah! I have done great injustice to myself and none except You forgives sins, so please forgive me and be merciful to me as You are the Forgiver, the Merciful).)

There are many other Hadiths on this subject.