Translation
(21) Actions while Praying
كتاب العمل فى الصلاة
(10) Chapter: What kind of actions are permissible during As-Salat
(10) باب مَا يَجُوزُ مِنَ الْعَمَلِ فِي الصَّلاَةِ
Sahih al-Bukhari 1209 

Narrated Aisha:

I used to stretch my legs towards the Qibla of the Prophet (ﷺ) while he was praying; whenever he prostrated he touched me, and I would withdraw my legs, and whenever he stood up, I would restretch my legs.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، حَدَّثَنَا مَالِكٌ، عَنْ أَبِي النَّضْرِ، عَنْ أَبِي سَلَمَةَ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ كُنْتُ أَمُدُّ رِجْلِي فِي قِبْلَةِ النَّبِيِّ صلى الله عليه وسلم وَهُوَ يُصَلِّي، فَإِذَا سَجَدَ غَمَزَنِي فَرَفَعْتُهَا، فَإِذَا قَامَ مَدَدْتُهَا‏.‏
Reference : Sahih al-Bukhari 1209
In-book reference : Book 21, Hadith 13
USC-MSA web (English) reference : Vol. 2, Book 22, Hadith 300
  (deprecated numbering scheme)