Two supplications are never rejected, or are rarely rejected: supplication made upon hearing the call to prayer and during battle when the two armies are engaged in fighting. [Abu Dawood]

«ثنتان لا تُرَدَّانِ، أو قلما تردان: الدعاء عند النداء وعند البأس حِينَ يُلْحِمُ بَعْضُهُ بَعْضًا». [صحيح.] - [رواه أبو داود.]