Musnad Abu Ya'la al-Mawsili 150:274

Masruq ibn al-Marzuban narrated to us, Ibn Abi Za'idah narrated to us, Muhammad ibn 'Amr narrated to us, from his father, from 'A'ishah, who said: When the revelation would descend upon the Prophet, may Allah bless him and grant him peace, he would experience what Allah, the Mighty and Majestic, said: "Indeed, We will cast upon you a heavy word."