Authentic Supplication Number 67.

Sah’l bin Sa`d  said: The Messenger of Allah  said: “Two are not turned down, or are rarely turned down: supplications at the time of the call [to prayer], and at war time when they entangle one another.”

Reported by Abu Dawood. Al-Albani said it was comely-sound, which, he says, agrees with Ibn Hajar’s opinion in his An-Nata’ij. Al-Albani included this hadeeth in The Authentic of Good Sayings as #60.