Abd al-Rahman bin Abi Layla (may Allah have mercy on him) says: I was with Sayyiduna al-Bara' bin Azib and Sayyiduna Umar bin al-Khattab (may Allah be pleased with them both) at al-Baqi'. They looked towards the crescent, and meanwhile a rider came. Sayyiduna Umar (may Allah be pleased with him) met him and asked him: Where have you come from? He said: From the west. Sayyiduna Umar (may Allah be pleased with him) said to him: Have you seen the crescent? He said: Yes. Sayyiduna Umar (may Allah be pleased with him) said: 'Allahu akbar' (Allah is the Greatest); indeed this one man suffices the Muslims. Then Sayyiduna Umar (may Allah be pleased with him) stood up, performed ablution, wiped over his leather socks, and then led the Maghrib prayer. Then he said: I saw him (peace and blessings be upon him) do just this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8193