Riyad as-SalihinChapter 11Hadith 646
Full Sharh — Riyad as-Salihin 646
وعن سهل بن سعد رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم "ثنتان لا تردان، أو قلما تردان: الدعاء عند النداء وعند البأس حين يلحم بعضهم بعضًا" ((رواه أبو داود بإسناد صحيح)).
English Translation
Sahl bin Sa'd (May Allah be pleased with him) reported:: The Messenger of Allah (ﷺ) said, "Supplications at two times are never turned down (or said, "Are seldom turned down"), a supplication after the Adhan has been proclaimed, and a supplication during the battle combating the enemy.".
Source
Riyad as-Salihin(رياض الصالحين)
The Book of Jihad
Hadith Number
646