If purifying the heart is so important, why didn’t the earliest Muslims have this whole system of tasawwuf? It feels like something added later, and that scares me as a possible innovation.

The fear behind this question deserves a plain answer. The thing you are afraid tasawwuf might be, a novelty invented later, is the opposite of what it is. The reality at its centre, the purification of the heart, is as old as the Revelation. It came down with the first verses.

Look at how the Prophet’s own mission was described. When Allah tells us why He sent His Messenger, He names purification as one of the very reasons, standing alongside the teaching of the Book.

“It is He who sent among the unlettered a Messenger from themselves, reciting to them His verses, purifying them, and teaching them the Book and wisdom.”

Qur’an, Al-Jumu’ah 62:2 · meaning

That word yuzakkeehim, He purifies them, is the whole substance of tasawwuf. This purifying of hearts was not an afterthought of later generations. It was among the first duties for which the Messenger himself was raised up. The Companions did not read of it in a manual; they received it directly, sitting in his blessed company, their hearts polished by his gaze and his teaching. The same purpose is stated again elsewhere.

“a Messenger from among yourselves, reciting to you Our verses and purifying you and teaching you the Book…”

Qur’an, Al-Baqarah 2:151 · meaning

So the earliest Muslims did have this, and had it in its purest and richest form. What they did not yet have was the later name and the organized method, the arranged lessons and gentle disciplines that took shape only once the living presence of the Prophet was no longer among the people and hearts needed a settled way to keep receiving that same polishing.

Consider how the sciences you already trust came to be. The Companions recited the Qur’an with perfect beauty long before anyone wrote down the rules of tajweed, and they reasoned soundly from the sources long before the principles of usul al-fiqh were set in order. The naming and the system came later; the reality was there from the first day. Tasawwuf is the same: an old light given a lantern, so that it could be carried safely to hearts far from that first gathering.

One small step: Choose one short verse you love and recite it slowly today, pausing at its meaning and asking Allah to purify your heart through it, just as the Messenger purified the hearts of those who sat before him.