“The key to a Christian conception of studies is the realization that prayer consists of attention. It is the orientation of all the attention of which the soul is capable toward God. The quality of the attention counts for much in the quality of the prayer. Warmth of heart cannot make up for it.”

Too Busy Not to Pray (2008, InterVarsity Press)

No. 29.
Seventy Resolutions (1722-1723)

The Writings of Marguerite Bourgeoys, p. 169

Waiting on God (1950), Reflections on the Right Use of School Studies with a View to the Love of God

This has similarly been attributed to Buchan, but is actually a misrendering of a sentence from the first paragraph of John Bunyan, Discourse on Prayer. Bunyan's original sentence reads: "It is the opener of the heart of God, and a means by which the soul, though empty, is filled."
Misattributed

Letter to Jonathan Sewall (October 1759)
1750s
Context: Tis impossible to judge with much Præcision of the true Motives and Qualities of human Actions, or of the Propriety of Rules contrived to govern them, without considering with like Attention, all the Passions, Appetites, Affections in Nature from which they flow. An intimate Knowledge therefore of the intellectual and moral World is the sole foundation on which a stable structure of Knowledge can be erected.

Vol. II, p. 31
1980s, Letters to the Schools (1981, 1985)
Context: Attention is this hearing and this seeing, and this attention has no limitation, no resistance, so it is limitless. To attend implies this vast energy: it is not pinned down to a point. In this attention there is no repetitive movement; it is not mechanical. There is no question of how to maintain this attention, and when one has learnt the art of seeing and hearing, this attention can focus itself on a page, a word. In this there is no resistance which is the activity of concentration. Inattention cannot be refined into attention. To be aware of inattention is the ending of it: not that it becomes attentive. The ending has no continuity. The past modifying itself is the future — a continuity of what has been — and we find security in continuity, not in ending. So attention has no quality of continuity. Anything that continues is mechanical. The becoming is mechanical and implies time. Attention has no quality of time. All this is a tremendously complicated issue. One must gently, deeply go into it.

Ibid.
The Education of the Stoic

Husayn al-Nuri al-Tabarsi, Mustadrak al-Wasā'il, vol. 11, p. 323
Regarding Knowledge & Wisdom, Religious