It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would stand in prayer at night for so long that his blessed feet would crack."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 5003
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ق ت ن هـ] عن المغيرة. الروض ٦٢٩، ٧٩٦.