Abu al-Zinad, Rabi'ah and Abu al-Nadr, all three without any disagreement among them, state that the Messenger of Allah (peace and blessings be upon him), Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them both) were lowered into the niche grave from the side of the head. The Shaykh says: This practice is well known among the people of the Hijaz.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 7055