Imam Ibn Rajab Al-Hanbali (d. 795 AH), may Allah be pleased with him, noted further on this topic:
There is also the handed down narration from Az-Zuhri in which the Prophet, peace and blessings of Allah be upon him, is reported to have said, “Whoever said some 10,000 times in one day the following supplication, Allah shall free him from the Fire just as if he came with the restitution of 10,000 people he killed and it was accepted from him:
There is no god but Allah! He is One and Unique without partner for Him.”
(4) another benefit of 9 Dhul Hijjah is that if he should free a slave when possible, then he will be freed from punishment. This is because whoever freed a believing slave, Allah shall free every limb of that individual from the Fire.
Hakim ibn Hizam, may Allah be pleased with him, used to stop at `Arafah and he had with him 100 cows covered in garlands and 100 slaves that he would free and he would address the people and they would answer from weeping and supplication:
Our Lord! This is Your Slave and he has freed one of his slaves. We in turn are Your Slaves so free us as well!
Harun Ar-Rashid used to have a similar procession. Abu Qilabah used to free one female servant on the `Eid ul-Fitr hoping that this would be a means of his being manumitted from the Fire.
(5) Making much supplication for forgiveness and manumission as it is the hope for the supplication to be answered in that vein. It was collected by Ibn Abid-Dunya with his chain of transmission from `Ali who said that there is no day in the earth except that Allah possesses people that He has manumitted therein; and there is no day with more manumissions in it for the necks of slaves than the day of `Arafah.
And the one who is in that day should definitely say at the time:
Allah! Free my neck from the Fire. Make vast for me the permissible sustenance, turn away from the me the sinful among the Jinn and people as this is the general nature of my supplication today!
Source: Lata’if ul-Ma`arif Fima Li-Mawasim il-`Ami Min Al-Waza’if, pp. 308-309