Ramana Maharshi:

Know well that even performing tapas (spiritual practice) and yoga with the intention ‘I should become an instrument in the hands of the Lord Siva’ is a blemish to complete self-surrender, which is the highest form of being in His service.
(Guru Vachaka Kovai, verse 471)

Sri Sadhu Om’s Comments:

Since even the thought ‘I am an instrument in the Lord’s hand’ is a means by which the ego retains its individuality, it is directly opposed to the spirit of complete self-surrender, the ‘I’-lessness. Are there not many good-natured people who engage themselves in prayers, worship, yoga and such virtuous acts with the aim of achieving power from God and doing good to the world as one divinely commissioned? It is exposed here that even such endeavours are egotistical and hence contrary to self-surrender.