The Messenger of Allah ﷺ used to pray until his feet became cracked, meaning split open.
أخبرنا عمرو بن علي قال حدثنا صالح بن مهران وكان ثقة قال حدثنا النعمان بن عبد السلام عن سفيان عن عاصم بن كليب عن أبيه
كان رسول الله ﷺ يصلي حتى تزلع يعني تشقق قدماه
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[Sahih]. It is also found in Sunan al-Kubra (Hadith: 1326), and Dulabi narrated it from Nasai's route in al-Kuna (1/200), but he did not say, "Ahmad bin Shuayb Nasai narrated to us," rather he said, "Amr bin Ali, meaning Fallas, narrated to us," and so on. Sufyan here is either Thawri or Ibn Uyayna. Iraqi said its isnad is good, and the hadith has many corroborating narrations, among them the preceding hadith.