Abdullah bin Abi Qatadah (may Allah have mercy on him) narrates from his father (may Allah be pleased with him): We think that he (peace and blessings be upon him) lengthened the first rak'ah so that the people could catch the first rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2486