3:27  

प्रकृतेः क्रियमाणानि गुणैः कर्माणि सर्वशः ।
अहंकारविमूढात्मा कर्ताहमिति मन्यते ॥३- २७॥

prakRRiteH kriyamaaNaani guNaiH karmaaNi sarwashaH ।

ahaMkaarawimooDhaatmaa kartaahamiti manyate ॥३- २७॥

Source: Automatic ITRANS Transliteration

In nature obligations by qualities are fulfilled, all.
Egotistical, foolish souls think "I am the doer."

Foolish souls in their egotism think, "I am the agent," but in nature, all obligations are fulfilled by the qualities.

Source: Milind S. Pandit

All works are being done by the Gunas (or the energy and power) of nature, but due to delusion of ego people assume themselves to be the doer. (See also 5.09, 13.29, and 14.19)

Source: The Bhagavad-Gita: The Sacred Song

The bewildered spirit soul, under the influence of the three modes of material nature, thinks himself to be the doer of activities, which are in actuality carried out by nature.

Source: Bhagavad Gita As It Is