Abd al-Rahman bin Abi Layla narrates that Sayyiduna Umar (may Allah be pleased with him) accepted the testimony of a single man for sighting the crescent of Eid al-Fitr or al-Adha.
Muhammad bin Ali al-Warraq said: I asked Abu Nu'aym: Did Ibn Abi Layla hear from Sayyiduna Umar (may Allah be pleased with him)? He said: I do not know. I asked Yahya bin Ma'in: Did Ibn Abi Layla hear from Umar? He did not affirm it. Ali (al-Daraqutni) said: Abd al-A'la is Ibn Amir al-Tha'labi; others are more reliable than him, and the hadith of Abu Wa'il from Umar has a sounder chain than it; al-A'mash and Mansur narrated it from Abu Wa'il.
Yahya bin Ma'in was asked about Abd al-Rahman bin Abi Layla narrating from Umar, and he said: He did not see him. I said to him: What about the hadith he narrates: "We were with Umar looking for the crescent"? He said: "It is nothing."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8195