It was narrated that Mustawarid bin Shaddad said:"I saw the Messenger of Allah performing ablution, and he ran his little finger between his toes." (Sahih) Another chain with similar wording
Hadith No. 446
Sunan Ibn Majah
Gradings:
Al-Albani:
Sahih
Muhammad Fouad Abd al-Baqi:
Sahih
Shuaib Al Arnaut:
Sahih Lighairihi
Zubair Ali Zai:
Sahih