Yahya bin Abi Ishaq said: I heard Sayyiduna Anas (may Allah be pleased with him) say: "We set out with the Messenger of Allah (peace and blessings be upon him), shortening (the prayer), until he reached Makkah. We stayed there ten (days), and he kept shortening until he returned."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5457