Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated: I stood up and began to pray two rak‘ahs while the iqamah for the (obligatory) prayer had been given. So the Messenger of Allah (peace and blessings be upon him) said: "Will you pray four rak‘ahs for the Fajr prayer?"
Hadith Referenceمسند اسحاق بن راهويه / كتاب الصلوٰة / 200
Hadith Takhrij«مسند احمد : 354/1 قال شعيب الارناوط : اسناده حسن .»