SB 10.69.15

tasyāvanijya caraṇau tad-apaḥ sva-mūrdhnā
 bibhraj jagad-gurutamo ’pi satāṁ patir hi
brahmaṇya-deva iti yad guṇa-nāma yuktaṁ
 tasyaiva yac-caraṇa-śaucam aśeṣa-tīrtham
Synonyms: 
tasya — his; avanijya — washing; caraṇau — the feet; tat — that; apaḥ — water; sva — His own; mūrdhnā — on the head; bibhrat — carrying; jagat — of the entire universe; guru-tamaḥ — the supreme spiritual master; api — even though; satām — of the saintly devotees; patiḥ — the master; hi — indeed; brahmaṇya — who favors the brāhmaṇas; devaḥ — the Lord; iti — thus called; yat — since; guṇa — based on His quality; nāma — the name; yuktam — fitting; tasya — His; eva — indeed; yat — whose; caraṇa — of the feet; śaucam — the bathing; aśeṣa — complete; tīrtham — holy shrine.
Translation: 
The Lord bathed Nārada’s feet and then put the water on His own head. Although Lord Kṛṣṇa is the supreme spiritual authority of the universe and the master of His devotees, it was proper for Him to behave in this way, for His name is Brahmaṇya-deva, “the Lord who favors the bṛāhmaṇas.” Thus Śrī Kṛṣṇa honored the sage Nārada by bathing his feet, even though the water that bathes the Lord’s own feet becomes the Ganges, the ultimate holy shrine.
Purport: 

Since Lord Kṛṣṇa’s own lotus feet are the source of the most holy Ganges, the Lord did not have to purify Himself by bathing Nārada Muni’s feet. Rather, as Śrīla Prabhupāda explains: “Lord Kṛṣṇa in Dvārakā enjoyed the pastimes of a perfect human being. When, therefore, He washed the feet of the sage Nārada and took the water on His head, Nārada did not object, knowing well that the Lord did so to teach everyone how to respect saintly persons.”