Qatadah (may Allah have mercy on him) said that Sayyiduna Anas bin Malik (may Allah be pleased with him) narrated to him that the Prophet (peace and blessings be upon him) said: "I stand in prayer intending to make the recitation long, then I hear the sound of a child crying, so I shorten the prayer so that it does not become hard on his mother."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4042