Shortening the prayer for a crying child

أَنَسَ بْنَ مَالِكٍ، حَدَّثَهُ أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ: إِنِّي لأَدْخُلُ فِي الصَّلاَةِ وَأَنَا أُرِيدُ إِطَالَتَهَا، فَأَسْمَعُ بُكَاءَ الصَّبِيِّ، فَأَتَجَوَّزُ فِي صَلاَتِي مِمَّا أَعْلَمُ مِنْ شِدَّةِ وَجْدِ أُمِّهِ مِنْ بُكَائِهِ.‏

Hazrat Anas bin Malik narrated, that “The Holy Prophet^sa said, ‘I sometimes begin the prayer intending to prolong it, but when I hear a child crying, I shorten the prayer, knowing the anguish his mother feels because of his crying.’” 

(Sahih al-Bukhari, Kitab al-adhan, Bab man akhaffa s-salata ‘inda buka’i s-sabiyy, Hadith 709)