“Is anyone of you incapable, when he prays, of stepping forwards or backwards, or to his right or left?” meaning in order to offer a voluntary prayer.
عَنِ النَّبِيِّ ﷺ قَالَ أَيَعْجِزُ أَحَدُكُمْ إِذَا صَلَّى أَنْ يَتَقَدَّمَ أَوْ يَتَأَخَّرَ أَوْ عَنْ يَمِينِهِ أَوْ عَنْ شِمَالِهِ يَعْنِي السُّبْحَةَ