Why are prayer and zakat always mentioned together?

The word "Zakat" is mentioned 29 times in the Holy Quran, all of them preceded by the word "Salah" (prayer), except in two instances: "And My mercy encompasses all things. So I will decree it [especially] for those who fear Me and give zakat and those who in Our verses believe" [Al-A'raf:156], and "Those who do not give zakat, and in the Hereafter they are disbelievers" [Fussilat:7]. This close association indicates the importance of Zakat, for Salah is the pillar of religion, and Zakat is usually mentioned after it due to its great significance. Abu Bakr As-Siddiq, may Allah be pleased with him, understood this importance and fought against those who withheld Zakat. Abu Hurairah narrated that when the Messenger of Allah, peace and blessings be upon him, passed away and some Arabs apostatized, Umar said to Abu Bakr: "How can you fight people when the Messenger of Allah, peace and blessings be upon him, said: 'I have been commanded to fight people until they say 'La ilaha illa Allah' (there is no god but Allah). So whoever says it, his wealth and life are protected from me, except by its right, and his accountability is upon Allah'?" Abu Bakr replied: "By Allah, if they withhold a rope they used to give to the Messenger of Allah, peace and blessings be upon him, I would fight them for withholding it." Just as the mention of Zakat is consistently linked with the mention of Salah in the Quran, this linkage is also found in the Sunnah of the Prophet, such as in the phrase: "Establishing Salah and giving Zakat."