It is a heart that accepts what Allah has decreed (redha), be it a little or a lot, good or bad. A heart that is not thankful with a little will not be thankful with a lot. The Prophet also said that whosoever is not thankful to people is not thankful to God. People are merely mortals through whom God grants us favours. Whatever they give, in whatever amount, and whether they give at all, is up to Allah.
If we look within and around us there is plenty to be thankful for – indeed the favours of Allah are many. In the Quran Allah dares us to count them all, but we will never be able to number them. Yet little thanks we give, and what we seem to do is complain about the small things that make us unhappy. If we were truly grateful we would appreciate all the blessings that Allah has bestowed upon us, and this appreciation is beyond the tongue reciting ‘alhamdulillah’ (Praise be to Allah). It is the heart submitting to His will, obedient of His orders and patient with His tests. It is the heart that has complete faith in His Mercy.
Allah says that if we are thankful He will give us more, but if we are ungrateful it would only incur His displeasure. The great companion of the Prophet SAW, Ali (RA) also once said: "Do not let your thanklessness drive away even the least of your blessings." When we hear this we seem to only think of how nice things become abundantly more, or otherwise disappear, but the hidden meaning here is that when received with thankful acceptance, even the most painful calamity serves only as something that draws us closer to Him, elevates our station and increases our iman. When received with thanks God's tests open doors to more of His favours.
But even at the start of Man's creation Satan knew: "Then I will certainly come to them from before them and from behind them, and from the right-hand side and from the left-hand side, and You (Allah) will not find most of them thankful." (7:17). Allah also says n Surah al-Mulk: "Say it is He Who has created you, and endowed you with hearing (ears), seeing (eyes), and hearts. Little thanks you give". Perhaps thankful hearts are few and far between?
We ask Allah for a thankful heart.