Allah, exalted be His praise, says: "Detain them both after the prayer, then let them both swear by Allah." [Surah al-Ma'idah: 106]
Imam al-Shafi'i (may Allah have mercy on him) said: "And the commentators have said: (It refers to) the Asr prayer."
The Shaykh (Imam al-Bayhaqi) (may Allah have mercy on him) said:
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / Q20700
Abu Musa al-Ash'ari (may Allah be pleased with him) said concerning the story of the will: This matter has not happened since the time of the Prophet (peace and blessings be upon him). Then he made the two of them swear after Asr that they had not acted treacherously.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20700
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "There are three people to whom Allah will not speak, nor will He purify them, and for them is a painful punishment: 1. A man who withholds surplus water from travellers. 2. A man who pledges allegiance to an imam for the sake of the world: if he gets the world he fulfils his pledge, otherwise he does not fulfil it. 3. One who bargains over goods after Asr and swears by Allah, saying: 'I was given such-and-such for it,' and the other believes him." The hadith of Waki' does not contain: 'the man who pledges allegiance to an imam.'
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20701
Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "There are three people to whom Allah will not speak, nor will He purify them, nor will He look at them with mercy: (1) one who, after Asr, swears an oath and usurps the property of a Muslim brother; (2) a man who says: 'I am being offered a higher price for my goods than what you are giving,' while he is lying; (3) one who withholds surplus water. Allah Almighty will say: 'I withhold My bounty from you just as you withheld surplus water, though you had not worked for it.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20702
Ibn Abi Mulaykah (may Allah have mercy on him) said: I wrote to Ibn Abbas (may Allah be pleased with them both) about two girls, one of whom had struck the other, and there was no witness either. He replied: Detain them after Asr, then recite this verse to them: "Indeed, those who exchange the covenant of Allah and their oaths for a small price." [Surah Al Imran: 77] He said: I did so, and she confessed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20703
Mutarrif bin Mazin narrates with his chain, though I do not remember it, that Sayyiduna Ibn al-Zubayr (may Allah be pleased with him) said: Take the oath on the Qur'an.
Imam al-Shafi'i (may Allah have mercy on him) said: I saw Mutarrif in San'a taking oaths on the Qur'an, and the governors of the regions also used to administer oaths on the Qur'an, and in my view that is good.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20704