قُلْ لَا أَمْلِكُ لِنَفْسِي ضَرًّا وَلَا نَفْعًا إِلَّا مَا شَاءَ اللَّهُ
Qul lā amlikku linafsī dharrawn-walā naf‘an illā mā shā’a-llāhu
Say: I have no control over my harms or any benefits except what Allah pleases (Sūrat Yūnus, No. 10, Āyat 49)
Human beings often see themselves as in charge of most of the things they do in life, or would like to believe that they have the ability to do so. As believers we acknowledge that there is a Creator and Lord of the universe. But in our everyday affairs, we are prone to consider ourselves as having the ability to benefit or harm ourselves. We attribute our successes and failures to ourselves, or to other people and causes around us.
In the verse above, Almighty Allah tells His Messenger to declare that he possesses no control over what benefits or harms him, except that part of the control which Allah wishes to give him. So even if we are able to control some part of the gain or loss for ourselves, it is only because Allah has chosen to give that much to us. If He had wished, He would not have given it to us (at all). And if He wishes, He can take it away from us. This understanding is part of the true essence of Tawhīd. It is the Tawhīde Af‘ālī, i.e. the Tawhīd in actions. This is the belief that all actions take place by the Will of God; He is the cause of all things, seen and unseen. No action or event exists on its own, independent of the will of Allah. The true cause of all effects is only Allah.
The sincere acceptance of the control of Allah over everything makes a believer purely monotheistic. It is very easy to be a believer in one God with traces of polytheism. Every Prophet emphasized his dependence on God as the Primary cause of all effects. It can be seen in the prayers of:
Nabī Ibrāhīm: O our Lord! Surely I have settled a part of my offspring in a valley unproductive of fruit near Thy Sacred House, our Lord, that they may keep up prayer; therefore make the hearts of some people yearn towards them and provide them with fruits. (Sūrat Ibrāhīm, 14:37)
Nabī Mūsā: O my Lord! Expand my chest for me, and make my affair easy to me, and loosen the knot from my tongue. (Sūrat Tāhā, 20:25-27)
Nabī ‘Īsā: O Allah, our Lord! Send down to us food from heaven which should be to us an ever-recurring happiness, to the first of us and to the last of us, and a sign from You, and grant us means of subsistence, and You are the best of the Providers. (Sūratul Mā’idah, 5:114)
Recite this verse often to enhance your recognition of Allah as the Primary cause of all effects. It will strengthen your complete dependence on him, and allow you to hold on to a firm handle that never breaks
فَمَنْ يَكْفُرْ بِالطَّاغُوتِ وَيُؤْمِنْ بِاللَّهِ فَقَدِ اسْتَمْسَكَ بِالْعُرْوَةِ الْوُثْقَىٰ لَا انْفِصَامَ لَهَا ۗ وَاللَّهُ سَمِيعٌ عَلِيمٌ
So whoever rejects the taghūt and believes in Allah hath grasped the most trustworthy hand-hold, that never breaks (Sūratul Baqarah, 2:256)
Sources: Āytaullāh Nāsir Makārim Shirāzī (ed), Tafsīr-e Namūneh.