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