Thomas Boston, Works, 1:543: "Pray earnestly for the Spirit, Luke 11:13. God has made a promise of the Spirit, and gives that as a ground of your prayer for him; Ezek. 36:27, 37, “A new Spirit will I put within you… I will yet for this be inquired of by the house of Israel, to do it for them.” And although God regard not prayer as performed by one without the Spirit, yet he regards it as a means and ordinance of his own appointment, whereby the Spirit is conveyed into the hearts of his elect."
Works, 2:561-562: "Objection. But it is needless for them to pray, since they cannot pray acceptably. Answer. No: for it is a mean of grace, and an ordinance of God; and though God have no respect to it as it is their performance, yet he may have respect to it as it is his own ordinance, and do good to them by it. The matter lies here: they are neither to continue in their sinful state, nor to satisfy themselves with their praying in that condition, but come out of it, and join themselves to God’s family, and so they will come to pray acceptably."
Works, 2:561-562: "Objection. But it is needless for them to pray, since they cannot pray acceptably. Answer. No: for it is a mean of grace, and an ordinance of God; and though God have no respect to it as it is their performance, yet he may have respect to it as it is his own ordinance, and do good to them by it. The matter lies here: they are neither to continue in their sinful state, nor to satisfy themselves with their praying in that condition, but come out of it, and join themselves to God’s family, and so they will come to pray acceptably."