Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says: Whenever the Prophet (peace and blessings be upon him) swore an oath, he would say: "By the One in whose hand is the soul of Abu al-Qasim."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19811