Majma al-Zawaid — Hadith 14866

Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: On the day of Uhud, the Prophet (peace and blessings be upon him) said to Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him): "Take on the flesh of the enemy" (that is, attack them). So Sayyiduna Sa'd (may Allah be pleased with him) placed an arrow in his bow and said: "O Allah! This is Your arrow, and it is in Your path. O Allah, help Your Messenger." The Prophet (peace and blessings be upon him) said: "O Allah! When Sa'd supplicates to You, answer his supplication."
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains Abu Sa'd al-Baqqal, who is a mudallis and thiqah (trustworthy). This hadith is strengthened by the two hadiths that have already passed in the chapter on Sayyiduna Sa'd's (may Allah be pleased with him) supplications being answered.